Exodus 18:13

And the day after, the day of reconciliation, Mosheh sat to judge the people: and the people stood before Mosheh from morning till evening.

AND IT CAME TO PASS ON THE MORROW, i.e., on the morrow of the day on which they held the above-mentioned [feast], THAT MOSES SAT TO JUDGE THE PEOPLE. In the Mechilta, the Rabbis have said: “On the morrow. That is, on the morrow after the Day of the Atonement.” Now the intent of the Rabbis was not that on the morrow alludes to the Day of Atonement, for Scripture has not mentioned the Day of Atonement at all that it should now refer to it by saying on the morrow thereof. Nor is the term on the morrow to be understood literally, [i.e., the day after the Day of Atonement], for they did not eat on the Day of Atonement, that is, if they observed such a day in the first year before they were commanded concerning it. Moreover, it was on the Day of Atonement that the second Tablets of the Law were given. On the following day, Moses came and he spoke to the children of Israel, and he gave them in commandment all that the Eternal had spoken with him in Mount Sinai. It thus could not have been a day on which he sat to judge the people, when the people stood about him from the morning unto the evening. It is also impossible to say that this was on the morrow of the Day of Atonement of the second year, for after the Israelites journeyed [from Sinai on the twentieth day of Iyar in the second year (Numbers 10:11)], Hobab [i.e., Jethro], said, I will depart to mine own land, and to my kindred.
Rather, the intent of this Beraitha, [i.e., the Mechilta quoted above, “on the morrow after the Day of Atonement”], is that it was some day after the Day of Atonement, since Moses had no free day on which to sit in judgment from the day they came to Mount Sinai until after the Day of Atonement of that first year.

לשפוט את העם. Even if we accept the opinion that Jethro had arrived at the encampment of the Jewish people before the revelation at Mount Sinai, mundane matters had been subject of judgment all the time. Any disagreement concerning people’s property needed adjudication. While it is true that at Marah, prior to the revelation at Sinai, not only social laws but also some ritual laws had been revealed (Exodus 15,25), the opinion that Yitro arrived after the revelation at Mount Sinai is more plausible seeing that we read here of the Israelites being encamped at the Mountain of G’d (verse 5). Moreover, chapter 19 commences with the words: “In the third month after the Exodus on the first of the month the Israelites entered the desert of Sinai after having journeyed from Refidim.” It is clear from there that what happened at Refidim and the encampment at Mount Sinai occurred before what is discussed in our paragraph, but that this paragraph was inserted here in order not to interrupt the portions dealing with all the commandments which commence in chapter 21 after the revelation at Mount Sinai.

מן הבקר עד הערב, seeing he was the only judge and he had no assistants.

ויעמוד העם על משה, waiting till he would find time to address their individual problems after he had finished dealing with the collective problems pertaining to the whole community and had finished listening to the people’s dignitaries.

ויהי ממחרת, “It happened on the following day, etc.” according to Rashi, this was the day following Yom Kippur, on which day Moses had returned from Mount Sinai for the third time and after G–d had forgiven the sin of the golden calf, and he had brought down with him a second set of Tablets inscribed by G–d Himself. Rashi is forced to explain it in this way as there had not been time between the revelation at Mount Sinai on the sixth day of Sivan, for Moses to have sat down to deal with the people’s personal problems, as on the day after the revelation he had been invited to the top of the mountain where he spent 40 days studying the Torah, and when he descended with the first set of tablets which he smashed when seeing the people dance around the golden calf, and his spontaneous return to the mountain to ask G–d to forgive the people seeing that they had cleansed themselves from their sin as best as they could, and the active idol worshippers had been executed. He had been invited to ascend the mountain again and bring with him a set of Tablets which he had carved out of the rock himself, and on which G–d had inscribed the text of the Ten Commandments a second time. These events have not been recorded in the Torah in their chronological order just as the events culminating in the Israelites departing from Mount Sinai have not been recorded in their chronological order; We read in Numbers 10,29 about Moses entreating Yitro to remain with the Jewish people, something that could not have occurred until the second month of the second year, as he speaks about the impending journey of the people and these are recorded as having broken camp on the twentieth day of that month. When Yitro is described as advising Moses to appoint delegates to function as judges of civil cases, this is reported in Exodus chapter eighteen, before the revelation had been reported. Moses could certainly not have sat down judging the people before the Torah had been revealed to him. Rashi therefore accepted the version of the Talmud tractate Zevachim folio 116, according to which Yitro had not arrived in the camp until after the revelation, i.e. after Yom Kippur, as how could Moses have had time to welcome him, eat with him, etc., between the revelation and the Day of Atonement, i.e. the tenth of Tishrey? Furthermore, as argued in the Tanchuma, G–d did not see why seeing the people had undergone hundreds of years of hardship in order to qualify for the revelation at Mount Sinai. Yitro who during all these years had lived tranquilly in Midian, should be granted the same privilege without having done much to earn it? [According to our tradition the privilege of being present at the revelation is one of three privileges that must be earned by having gone through יסורים, great hardships.